FX STX Superior Heavy or Slug Liner

Superior Heavy STX liner: for heavy pellets and slugsHuub Andriessen

If you're still shooting standard pellets, this liner is simply not for you, don't bother. The FX STX Superior Heavy Liner, also known as the Slug Liner, is specifically built for slugs, with tighter tolerances and refined rifling compared to the standard STX liner. That difference does translate to real-world accuracy gains when shooting slugs, but only if the rest of your setup supports it. Running slugs through a poorly tuned rifle won't give you the results you're after. My tip: if you're making the switch to slugs, combine this liner with the right probe and a slug-optimised power kit, otherwise you're leaving performance on the table.

FX STX Superior Smooth Twist X Heavy/Slug Liner

The FX Airguns Superior Heavy STX Liner is designed for shooters who want to optimize their FX air rifle for heavier pellets and slugs. Developed as the successor to the successful FX STX Slug Liner A, Slug Liner or Superior Slug liner,  it combines the proven Smooth Twist X manufacturing method with tighter production tolerances and an improved rifling profile.

Instead of cutting the rifling into the bore, FX forms the rifling by pressing it into the liner from the outside. This process creates an exceptionally smooth internal surface that reduces projectile drag and helps maintain consistent velocity and accuracy. The refined Superior manufacturing process allows for greater dimensional consistency from one liner to another.

The Superior Heavy version features a twist rate specifically selected to stabilize heavier projectiles. This makes it an excellent choice for high-power configurations using heavy pellets or slugs, where the correct projectile stabilization is essential for tight groups and dependable long-range performance.

A major advantage of the FX STX barrel system is its modular construction. The liner can be installed directly into a compatible FX STX barrel assembly without permanent modifications. This allows you to experiment with different calibers, barrel lengths and twist rates while retaining the same rifle platform.

For the best results, the rifle should be tuned to match the selected projectile, liner length and caliber. When converting to another caliber, additional calibre-specific components such as the correct pellet or slug probe, magazine and transferport  may also be required. You can find the Huma-Air Caliber Exchange Sets for the different FX Airgun models here

Key features

• Developed for heavy pellets and slugs
• Improved successor to the FX STX Slug Liner A, Slug Liner or Superior Slug liner
• Smooth Twist X rifling formed from the outside
• Refined production process with tighter tolerances
• Smooth internal finish for reduced projectile drag
• Interchangeable design for compatible FX STX barrel systems
• Available in multiple calibers and barrel lengths

Please note:

Several FX Heavy Liners will be replaced in 2026 by Superior Liners with a 1:16 twist rate, designed for both pellets and slugs. If you order a Heavy Liner for one of the models listed below, you will receive the corresponding Superior 1:16 Liner.

• 4.5 mm / .177 in 500 mm.
• 4.5 mm / .177 in 700 mm.

Available versions

• 4.5 mm / .177 in 500, 600, 700
• 5.5 mm / .22 in 380, 500, 600, 700 and 800 mm
• 6.35 mm / .25 in 500, 600, 700 and 800 mm
• 7.62 mm / .30 in 500, 600, 700 and 800 mm
• 9 mm / .35 in 700 and 800 mm

Please note: this product consists of the barrel liner only. Other barrel components, probes or tuning adjustments may be required depending on the rifle, caliber and intended projectile.